The present invention relates to electrophotographic devices, and in particular, to synchronization sensors and methods of generating synchronization signals based upon laser power.
In electrophotography, an imaging system typically sweeps a laser beam across a photoconductive surface in a scan direction as the photoconductive surface advances in a process direction that is orthogonal to the scan direction. The beam is modulated as it is swept so as to write one scan line for each sweep of the beam, thus forming a latent image on the photoconductive surface in a manner that corresponds with associated image data.
During laser scanning, a synchronization signal is generated by sweeping the laser beam across a synchronization sensor before the start of each scan line. This synchronization signal is used to align each scan line on the photoconductive surface and to compensate for scanning and optical variations from one scan line to the next.
Conventional synchronization sensors have fixed resistors that set the sensor gain based upon the expected laser power range and device operating points at which the laser is to be operated. However, variations in laser power, as may be desired to accomplish a predetermined electrophotographic effect, may cause the precise location at which the beam is sensed to shift due to the fixed power level required to trigger the synchronization sensor. As such, in a color printer with multiple scanning beams, each color plane may be shifted relative to the others and cause noticeable plane to plane mis-registration.
Some sensor systems seek to address the above problem by comparing the output of two photodiodes that are offset in the scan direction. This approach is only useful if the beams scanning across the sensor always move in the same direction relative to these sensors. This approach is also more costly than a single photodiode system. Other systems seek to address the above problem by boosting the output power of the laser beam to a uniform level during synchronization sensing. However, this may require that the imaging system set a first laser power for synchronization sensing, and a second laser power for imaging operations while sweeping the beam across an image area of the photoconductive surface.